This invention relates to thermal barrier coating systems for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a thermal barrier coating system which employs a ceramic layer and a diffusion aluminide bond coat incorporating an additive metal and an active element, which together promote the spallation resistance of the ceramic layer.
The operating environment within a gas turbine engine is both thermally and chemically hostile. Significant advances in high temperature alloys have been achieved through the formulation of iron, nickel and cobalt-base superalloys, though components formed from such alloys often cannot withstand long service exposures if located in certain sections of a gas turbine engine, such as the turbine, combustor and augmentor. A common solution is to provide such components with an environmental coating that inhibits oxidation and hot corrosion.
Coating materials that have found wide use for this purpose include diffusion aluminide coatings, which are generally single-layer oxidation-resistant layers formed by diffusion processes, such as a pack cementation process. Diffusion processes generally entail reacting the surface of a component with an aluminum-containing gas composition to form two distinct zones, the outermost of which is an additive layer containing an environmentally-resistant intermetallic represented by MAl, where M is iron, nickel or cobalt, depending on the substrate material. Beneath the additive layer is a diffusion zone comprising various intermetallic and metastable phases that form during the coating reaction as a result of diffusional gradients and changes in elemental solubility in the local region of the substrate. During high temperature exposure in air, the MAl intermetallic forms a protective aluminum oxide (alumina) scale or layer that inhibits oxidation of the diffusion coating and the underlying substrate.
For particularly high temperature applications, environmental coating systems often include a layer of thermal insulating ceramic over a diffusion coating, the latter of which is then termed a bond coat. The combination of the bond coat and ceramic layer is known in the art as a thermal barrier coating system. Various ceramic materials have been employed as the ceramic layer, particularly zirconia (ZrO2) fully or partially stabilized by yttria (Y2O3), magnesia (MgO), ceria (CeO2), scandia (Sc2O3), or another oxide. These particular materials are widely employed in the art because they exhibit desirable thermal cycle fatigue properties, and also because they can be readily deposited by plasma spray, flame spray and vapor deposition techniques.
A bond coat is critical to the service life of the thermal barrier coating system in which it is employed, and is therefore also critical to the service life of the component protected by the coating system. The oxide scale formed by a diffusion aluminide bond coat is adherent and continuous, and therefore not only protects the bond coat and its underlying superalloy substrate by serving as an oxidation barrier, but also chemically bonds the ceramic layer. Nonetheless, aluminide bond coats inherently continue to oxidize over time at elevated temperatures, which gradually depletes aluminum from the bond coat and increases the thickness of the oxide scale until such time as the scale reaches a critical thickness that leads to spallation of the ceramic layer at the interface between the bond coat and the aluminum oxide scale. Once spallation has occurred, the component deteriorates rapidly, and must be refurbished or scrapped at considerable cost.
In addition to excessive oxide growth, the ability of the bond coat to form and maintain a suitable aluminum oxide scale can be hampered by the interdiffusion of aluminum in the bond coat with the superalloy substrate, and from the diffusion of elements from the superalloy into the bond coat, such as during formation of the aluminide bond coat or during high temperature exposure. The diffusion and subsequent oxidation of elements such as molybdenum, tungsten, rhenium, titanium and tantalum within the aluminide bond coat can become thermodynamically favored as the aluminum within the coating is depleted through oxidation and interdiffusion. Furthermore, these elements tend to form voluminous, nonadherent scales that are deleterious to adhesion of the ceramic layer.
From the above, it is apparent that the service life of a thermal barrier coating system, and therefore the component protected by the coating system, is dependent on the bond coat used to anchor the thermal insulating ceramic layer. The prior art has proposed aluminide environmental coatings whose environmental resistance is improved by additions of various elements, such as platinum group metals, as disclosed in U.S. Pat. Nos. 3,677,789, 4,352,840 and 4,933,239. While improvements in oxidation have been achieved with these coatings, the requirements for an aluminide bond coat can differ from that of an aluminide environmental coating, since a bond coat must not only exhibit environmental resistance but also promote adhesion of the ceramic layer. For example, while oxide growth on an aluminide environmental coating promotes the environmental protection provided by the coating to its underlying substrate, continuous oxide growth on an aluminide bond coat is detrimental to the spallation resistance of its overlying ceramic layer. While U.S. Pat. No. 5,514,482 to Strangman suggests that diffusion aluminide bond coats may be modified with platinum, silicon, hafnium and oxides to promote scale adhesion, the problem of excess scale growth is neither recognized nor solved. Therefore, it would be desirable if improvements could be achieved for the service life of a thermal barrier coating system through the use of an improved bond coat that exhibits slower oxide scale growth.
It is an object of this invention to provide an improved thermal barrier coating system and process for a component designed for use in a hostile thermal environment, such as superalloy components of a gas turbine engine.
It is another object of this invention that the coating system includes a diffusion aluminide bond coat that is formed on the surface of the component.
It is still another object of this invention that the coating system includes a thermal insulating ceramic layer that is anchored to the component with an aluminum oxide scale on the surface of the aluminide bond coat.
It is yet another object of this invention that the aluminide bond coat is modified to produce a relatively slow-growing oxide scale that is relatively free of impurities and resists cracking and spalling.
It is a further object of this invention that the aluminide bond coat includes an additive metal and an active element, which together yield a bond coat characterized by increased resistance to oxidation as a result of the slower growth rate for the oxide scale, and further characterized by improved adhesion of the oxide scale to the bond coat, such that the spallation resistance of the ceramic layer is also promoted.
The present invention generally provides a thermal barrier coating system and a method for forming the coating system on a component designed for use in a hostile thermal environment, such as superalloy turbine, combustor and augmentor components of a gas turbine engine. The method is particularly directed to a thermal barrier coating system that includes an oxidation-resistant diffusion aluminide bond coat on which an aluminum oxide scale is grown to protect the underlying surface of the component and adhere an overlying thermal-insulating ceramic layer.
According to this invention, significant improvements in spallation resistance for the ceramic layer are achieved by forming the aluminide bond coat to include one or more specific metal additives, namely the noble metals (platinum, palladium and rhodium), chromium and/or silicon, and limited additions of the active elements yttrium and/or zirconium. The additive metal and active element constituents of the bond coat may be deposited prior to the aluminide bond coat, such as by sputtering or through a cathodic arc process. Alternatively, the active element constituent may be simultaneously deposited with the aluminide bond coat using pack cementation or gas phase processing. Once formed, the modified aluminide bond coat undergoes oxidation to form an aluminum oxide scale that serves as an oxidation-resistant barrier layer and chemically bonds the thermal-insulating ceramic layer.
According to this invention, appropriately alloying the aluminide bond coat to contain one or more of the above-noted additive metals and one or more of the above-noted active elements serves to significantly enhance the spallation resistance of the overlying ceramic layer by promoting the adherence of the aluminum oxide scale and slowing the growth of the oxide scale. The active elements improve oxide scale adhesion by altering the structure of the oxide, bond coat and oxide-bond coat interface and/or by tying up tramp elements such as sulfur. The additive metals generally promote the selective oxidation of aluminum at lower concentrations of aluminum in the bond coat. Furthermore, the noble metals impede the diffusion of refractory metals from the substrate so as to prevent their oxides from doping the oxide scale. Additive elements such as silicon and chromium react with refractory metals to form compounds that tie up the refractory metals, thereby preventing the formation of refractory metal oxides at the surface of the aluminide bond coat. As such, the additive metals provide advantages that are particularly notable where the component is a superalloy containing one or more refractory metals, such as tantalum, tungsten, molybdenum, titanium and rhenium. A synergistic effect appears to result due to the presence of both the additive and active elements, yielding a significantly slower oxide growth rate. Because stresses rise and adhesion declines as oxide scale thickness increases, the ultimate result being spallation, the slowed oxide growth rate provided by this invention is able to significantly extend the life of a thermal barrier coating system.
Other objects and advantages of this invention will be better appreciated from the following detailed description.